Air attacks alone will not be enough to deal with Isil now it controls so much of Syria and Iraq - and threatens the US and Europe - argues military historian

Is the West willing to live with the Islamic State governing and raising armies in the heart of theMiddle East? The question is not rhetorical. It is fundamental. If the answer is "no", then we must do what is needed to defeat the Islamic State in both Iraq and Syria.

Rejecting methods necessary to that end is effectively deciding to live with the Caliphate. We must not back into that decision through failure to consider the consequences of inaction.

Iraq and the US have defeated the Islamic State once before, in its previous incarnation as al-Qaeda in Iraq (AQI). AQI had established fortified defensive positions in and around Baghdad and the surrounding provinces, and all the way north to Mosul, by 2006.

A change in US strategy involving additional American forces, a rapid expansion of the Iraqi Security Forces, and the "Anbar awakening" in which Sunni tribes were mobilised against the extremists, coupled with important political changes in Baghdad, expelled AQI from almost all of its strongholds outside Mosul by mid-2009. We deprived AQI of the capacity to fight - but not the will.

AQI began its rehabilitation when Syria collapsed into civil war in 2011. The withdrawal of all American forces from Iraq at the end of 2011 - combined with the increasingly sectarian policies of Iraq's prime minister, Nuri al-Maliki - allowed AQI to rebuild in Iraq as well. It had re-established the ability to launch waves of suicide car bomb attacks against Baghdad in 2012 and began to regain control of lost territory in 2013.

The group, rebranded as the Islamic State of Iraq and al-Sham (Isis or Isil), swept into Mosul and south toward Baghdad earlier this year, effectively destroying two Iraqi Army divisions and scattering other Iraqi security forces elements.

Abu Bakr al-Baghdadi

Its leader, Abu Bakr al-Baghdadi, declared that he had established the Caliphate after the fall of Mosul, re-branding the group the Islamic State (IS). It remained on the offensive despite a rift with al-Qaeda's leader, Ayman al-Zawahiri, that led to the Islamic State's expulsion from al-Qaeda. It has consolidated control of the Euphrates River from Raqqa, Syria to Haditha, Iraq.

Even as the US began air strikes in Iraq, the Islamic State has conducted an offensive in Syria's Aleppo province in order to disrupt the critical supply routes that run from Turkey to the moderate opposition. It has demonstrated the ability to plan and conduct large-scale operations on several disparate fronts simultaneously. It does not appear to cease operations in one area when it comes under pressure in another.

The Islamic State is a determined, skilful, and lethal enemy. Defeating it again will require hard fighting and strong diplomacy. It will be a massive undertaking, both militarily and politically. It will certainly require a dramatic expansion of air strikes throughout Iraq and Syria.

It will also require the deployment of thousands of American forces - primarily Special Forces - to engage directly with moderate Sunnis in both countries and train the Iraqi Security Forces (ISF) and the Syrian moderate opposition.

Effective trainers must fight alongside those they are training in order to sustain their respect and trust. Americans must work directly with local tribes to help them survive against the lethal threat that the Islamic State poses.

In Syria, the US must help moderate Sunnis fight Bashar al-Assad while strengthening them against other violent Islamist forces as well. In Iraq we must once again help rebuild trust between the central government and the tribes.

None of these tasks can be accomplished from 30,000 feet.

American air strikes helped Iraqi and Kurdish forces retake the Mosul Dam, which sits close to Kurdistan in a remote, unpopulated area. That model will not work in heavily-populated Mosul or in the dense scrub of northern Babil or Diyala Provinces, however.

Driving the Islamic State from those areas will require hard fighting by well-trained and well-led military forces, not the militias and "volunteers" now swarming about them.

Failing to help rebuild and retrain those forces, and to support them directly, is likely to mean leaving large swaths of Iraq to the Islamic State. Failure to act in Syria will leave the Caliphate controlling the upper Euphrates River Valley.

The Islamic State has the resources of two cities, including technical elites, cash, radioactive material seized from research facilities in Mosul, and multiple oil fields. It was expelled from al Qaeda for being too aggressive. It has threatened the US and the West. Hundreds of European and American fighters have flowed through its ranks.

Left alone, it will attack both Europe and the US. It will be able to hurt us badly.

Reasonable people could nevertheless argue that the risk does not justify the cost of deploying thousands of troops, massive air power, all of our diplomatic resources, and large sums of cash to a region our peoples would rather forget. There are other threats and other challenges, to be sure, and the success of intervention is far from certain.

But those insisting on narrow options, inadequate to defeat the Islamic State, are effectively accepting that it will continue to rule and raise armies in Iraq and Syria. Strategies to contain and degrade those terrorist armies could be devised, but the armies and states would persist.

The problem is horrifically complex, the potential solutions are painful and dangerous. But the question we must answer is, unfortunately, very simple. Are we willing to live with the Caliphate - or not?

Prof Fred Kagan is resident scholar at the American Enterprise Institute, and a former professor of military history at US Military Academy, West Point